Doctorate thesis of University of Montpellier

Friday, November 17, 2017

 

Molecular basis of nitrate/cytokinin-dependent long-distance signaling in Arabidopsis thaliana

Arthur Poitout
BPMP, “Hormones, nutrients and development” team

 

Jury :
Mme Anne KRAPP, Institut Jean-Pierre Bourgin, UMR 1318 INRA-Agro,ParisTech Rapporteur
Mme Catherine RAMEAU, Institut Jean-Pierre Bourgin, UMR 1318 INRA-AgroParisTech, Rapporteur
Florian FRUGIER, Institute of Plant Sciences-Paris Saclay University (IPS2), Reviewer
Mme Virginie LAUVERGEAT, Ecophysiologie et Génomique Fonctionnelle de la Vigne, Reviewer
Patrick ACHARD, Institut de biologie moléculaire des plantes, Reviewer
Benoit LACOMBE, Biochimie et physiologie moléculaire des plantes, Thesis Director
Mme Sandrine RUFFEL, Biochimie et physiologie moléculaire des plantes, Thesis Supervisor

 

Abstract :
Plants are sessile organisms growing in a heterogeneous and fluctuating environment. Thus, foraging for nutrients is an important trait for plant growth and development. Nitrogen (N), especially as nitrate (NO3) form, is one limiting element for plant growth but is also highly mobile in the soil leading to frequent heterogeneity distribution. Plants are managing this constraint through the regulation of root development and NO3 uptake in the different parts of the root system according to the spatial NO3 availability and the N needs of the whole plant. This adaptation relies on a dual signaling pathway involving i) a local signaling related to external NO3 supply and ii) a root-shoot-root long-distance (systemic) signaling related to the plant N needs.
However, the molecular basis of the long-distance signaling as well as the regulatory mechanisms associated with, are not fully understood. They rely on the integration at the shoot level of signals originating from both NO3-supplied and N-deprived root parts. Therefore, the shoots have a key role for an efficient adaptation to heterogeneous NO3 environment through the adjustment of root physiology and development. Previously, cytokinin biosynthesis has been shown to be essential for both molecular and morphological root responses to NO3 heterogeneous environment. Moreover, it is known that upon NO3 supply, de novo biosynthesis of this hormone in the roots is increased along with its accumulation in the shoots. In this context, we hypothesized that cytokinins could correspond to an important root to shoot signal involved in NO3-dependent systemic signaling.
The main objective of my PhD project was to decipher and understand how the shoots control root NO3 acquisition in response to spatial NO3 heterogeneity. To do so, we used the ‘split-root’ system, in which physically isolated roots of a same plant are challenged with different NO3 environments. In this framework, we characterized physiological, metabolic and molecular responses of Arabidopsis wild-type plants that we compared to responses of mutants impaired in cytokinin biosynthesis, acropetal transport or perception. The combination of these different approaches allowed me to demonstrate that cytokinins, and especially trans-zeatin species are indeed a root to shoot messenger that is crucial for root responses to spatial NO3 heterogeneity. Moreover, I have shown that NO3 heterogeneous supply compared to homogeneous supply triggers a substantial reprogramming of gene expression in aerial part, which largely depends on this trans-zeatin transport toward the shoots. Finally, the integration of these transcriptomic modifications into gene networks led to the identification of interesting candidate genes to characterize the shoot-to-root signaling.